This application is based upon Japanese Patent Application Nos. Hei. 11-210805 filed on Jul. 26, 1999, and Hei. 11-212734 filed on Jul. 27, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to semiconductor physical quantity sensors for detecting a physical quantity such as acceleration or angular velocity, and particularly to a capacitance-detecting type semiconductor physical quantity sensor, wherein a moving electrode part and a fixed electrode part facing this moving electrode part are formed by forming trenches in a semiconductor layer of a supporting substrate consisting of a semiconductor and an applied physical quantity is detected on the basis of variation in a capacitance between these moving and fixed electrode parts.
2. Related Art
Semiconductor acceleration sensors include those of a differential capacitance type. The construction of a differential capacitance type semiconductor acceleration sensor of related art is shown in FIG. 17. A vertical sectional view on the line XVIIIxe2x80x94XVIII in FIG. 17 is shown in FIG. 18. In FIG. 18, a semiconductor substrate 102 is fixed to a package plate 100 with an adhesive 101, and a semiconductor thin film 104 is disposed on an insulating film 103 on the semiconductor substrate 102. Through holes 105, 106 are formed in the semiconductor substrate 102 and the insulating film 103 respectively. The semiconductor thin film 104 is patterned to form as separate sections a moving electrode bridge structure 107, a first fixed electrode cantilever bridge structure 108 and a second fixed electrode cantilever bridge structure 109, shown in FIG. 17. The moving electrode bridge structure 107 has anchoring parts 110, suspension parts 111, a weight part 112 and comb-shaped moving electrodes 113. The first fixed electrode cantilever bridge structure 108 has an anchoring part 114 and a fixed electrode 115. Similarly, the second fixed electrode cantilever bridge structure 109 has an anchoring part 116 and a fixed electrode 117. The moving electrodes 113 and the fixed electrodes 115, 117 face each other, and when acceleration is applied in the X-direction in FIG. 17, the weight part 112 displaces and a difference in capacitance between the moving electrodes 113 and the fixed electrodes 115, 117 changes, and by extracting this change in difference in capacitance as a voltage change it is possible to detect the acceleration.
However, when the temperature at which the sensor is being used changes, due to differences in the coefficients of thermal expansivity of the different parts of the sensor, that is, differences in coefficient of thermal expansivity between the package plate 100, the adhesive 101, the semiconductor substrate 102, the insulating film 103 and the semiconductor thin film 104, warp occurs in the semiconductor substrate 102. Because of this warp, as shown in FIGS. 19, 20A and 20B, the fixed electrode 117 (115) deforms, and the spacing d between the fixed electrode 117 (115) and the moving electrode 113 ceases to keep a constant value, as shown in FIG. 20B (d1xe2x89xa0d2). As a result, there has been the problem that the temperature characteristic of the sensor is poor.
Also, another semiconductor acceleration sensor of the capacitance-detecting type which has been proposed is shown in FIGS. 35 and 36. Here, FIG. 35 is a plan view and FIG. 36 a sectional view on the line XXXVIxe2x80x94XXXVI in FIG. 35. This sensor is formed by applying micro-machining using semiconductor manufacturing technology to a semiconductor substrate having an insulating layer J3 between a first semiconductor layer J1 and a second semiconductor layer J2.
In this semiconductor acceleration sensor, by forming trenches in the second semiconductor layer J2 of the semiconductor substrate, a moving electrode part J6 wherein a weight part J4 is integrated with projecting parts J5 is formed and comb-shaped fixed electrode parts J7, J8 facing the projecting parts J5 are formed. Here, the first semiconductor layer J1 and the insulating layer J3 constitute a supporting substrate, and an opening J9 open at the second semiconductor layer J2 side is formed in this supporting substrate. In the example shown in the drawings, the opening J9 is so formed as to pass right through the supporting substrate from the second semiconductor layer J2 side to the opposite side.
The moving electrode part J6 is elastically supported at both ends on the edge of the opening in the supporting substrate, and displaces over the opening J9 in the arrow X direction of FIG. 35 in correspondence with an applied acceleration. The fixed electrode parts J7, J8 are made up of facing electrodes J7a, J8a facing the projecting parts J5 of the moving electrode part J6 over the opening J9 and interconnection parts J7b, J8b fixed to the edge of the opening in the supporting substrate and supporting the facing electrodes J7a, J8a. Thus this related art semiconductor acceleration sensor is of a construction having at least one moving electrode part J6 and two fixed electrode parts, a first fixed electrode part J7 and a second fixed electrode part J8, provided on opposite sides of the moving electrode part J6.
Here, the capacitance between the facing electrode J7a of the first fixed electrode part J7 and the respective projecting part J5 of the moving electrode part J6 will be called the first detection capacitance CS1 and the capacitance between the facing electrode J8a of the fixed electrode part J8 and the respective projecting part J5 will be called the second detection capacitance CS2. In the drawings, the capacitances are shown with capacitor symbols. In correspondence with the displacement of the moving electrode part J6 caused by an applied acceleration, the detection capacitances CS1, CS2 change, and by detecting (differentially detecting) this as a difference of the detection capacitances CS1 and CS2, it is possible to detect the applied acceleration.
However, in studies carried out by the present inventors into the related art semiconductor acceleration sensor described above, the problem has arisen that manufacturing process error of the sensor causes the output error of the sensor, or offset, to be large. Next, a study carried out by the present inventors into this offset problem will be discussed on the basis of the related art sensor illustrated in FIGS. 35 and 36. FIG. 24A shows a detection circuit of a differential capacitance type semiconductor acceleration sensor. CP1, CP2 and CP3 denote parasitic capacitances.
Here, in this related art sensor, CP1 is the capacitance between the interconnection part J7b of the first fixed electrode part J7 and the supporting substrate, CP2 is the capacitance between the interconnection part J8b of the fixed electrode part J8 and the supporting substrate, and CP3 is the capacitance between interconnection parts J6b of the moving electrode part J6 and the supporting substrate. Also, J10 denotes a switched capacitor circuit (SC circuit); this SC circuit J10 has a capacitor J11 of capacitance Cf, a switch J12 and a differential amplifier circuit J13 and converts an inputted capacitance difference into a voltage.
An example of a timing chart of the circuit shown in FIG. 24A is shown in FIG. 24B. In this related art sensor, for example a carrier wave 1 (frequency 100 kHz, amplitude 0 to 5V) is inputted through a fixed electrode pad J7c and a carrier wave 2 (frequency 100 kHz, amplitude 0 to 5V) out of phase with a carrier wave 1 by 180xc2x0 is inputted through a fixed electrode pad J8c, and the switch J12 of the SC circuit J10 is opened and closed with the timing shown in the Figure. An applied acceleration is then outputted as a voltage value Vo as shown by the following expression Exp. 1:
Vo={(CS1xe2x88x92CS2)+(CP1xe2x88x92CP2)xc2x7CP3}xc2x7V/Cfxe2x80x83xe2x80x83Exp. 1
Here, V is the voltage across the pads J7c, J8c. Thus the output of the sensor is affected by the parasitic capacitances CP1, CP2 and CP3. Generally, if the area of two members constituting a capacitance C is written S and the spacing between them is written d, then C=xcex5xc2x7S/d. Therefore, when due to process dispersion or the like the area of the overlapping portions of the interconnection parts J7b, J8b and the supporting substrate changes, or the thickness of the insulating layer J3 in the supporting substrate varies and the spacing d regulating the capacitance C changes, dispersion arises in the parasitic capacitances CP1 and CP2, and CP1 and CP2 become unequal. That is, even when the applied acceleration is zero, a difference arises between the parasitic capacitances CP1 and CP2 and is outputted as offset.
Here, in this semiconductor acceleration sensor, normally, the opening J9 and the structures in which the moving electrode part J6 and the fixed electrode parts J7, J8 are released are formed by using photolithography and etching the semiconductor layer and the supporting substrate. Consequently, as causes of the area of the above-mentioned overlapping portions changing, in the process of etching the opening J9, [1] misalignment of a mask with respect to the supporting substrate, and [2] differences in etching progress during etching are conceivable.
FIGS. 37A and 37B are views illustrating process dispersion of the opening J9 resulting from these causes [1] and [2]. Cause [1], for example as shown in FIG. 37A, even if the opening J9 is formed to the predetermined shape, causes positional deviation in one direction, as shown with dashed lines. Consequently, the area of the above-mentioned overlapping portions is for example small on the capacitance CP1 side and large on the capacitance CP2 side.
And cause [2], for example as shown in FIG. 37B, causes the kind of shape deviation shown with dashed lines with respect to the target shape of the opening J9. Consequently, the area of the overlapping portions for example is small only on the capacitance CP1 side. In a study carried out by the present inventors, the deviations resulting from these causes [1] and [2] was xc2x11 to 50 xcexcm. And the thickness dispersion of the insulating layer J2 in the supporting substrate was xc2x10.1 xcexcm.
Thus, according to the studies carried out by the present inventors, in the differential capacitance type sensor described above, due to process dispersion of the sensor, positional deviation of the opening in the supporting substrate and shape deviation from its predetermined shape occur, and thickness dispersion of the insulating layer of the supporting substrate occurs. And it was found that because consequently the parasitic capacitances of the interconnection parts of the fixed electrode parts fixed to the edge of the opening in the supporting substrate vary, offset becomes large as a result.
And when along with increases in sensor sensitivity further studies were carried out into the problem of offset, it was found that the following problems occur. In the semiconductor acceleration sensor described above, as shown in FIG. 35, a moving electrode pad J6c connecting with the moving electrode part J6 and fixed electrode pads J7c, J8c connecting with the interconnection parts J7b, J8b of the fixed electrode parts J7, J8 are formed disposed substantially in a row on the same side of the opening in the supporting substrate.
One end of a respective wire J6d, J7d, J8d made of Al (aluminum) or Au (gold) is connected to each of the pads J6c, J7c and J8c, and the other ends of these wires are connected to an external circuit (not shown) including the above-mentioned SC circuit J10. Here, parasitic capacitances CW1, CW2 are formed between the wire J6d and the wire J7d and between the wire J6d and the wire J8d respectively. A detected circuit diagram obtained by adding these inter-wire parasitic capacitances to FIG. 24A and 24B is shown in FIG. 33. The applied acceleration is outputted as a voltage value Vo as shown by the following expression Exp. 2:
Vo={(CS1xe2x88x92CS2)+(CW1xe2x88x92CW2)+(CP1xe2x88x92CP2)xc2x7CP3}xc2x7V/Cfxe2x80x83xe2x80x83Exp. 2
Here, because parts of the above-mentioned wires other than the parts connected to the pads and the external circuit are movable they may vibrate, and as a result of this and positional deviations of the wire bonding the parasitic capacitances CW1 and CW2 differ greatly. Consequently, the problem arises that the parasitic capacitances CW1 and CW2 are not equal and offset dispersion among sensors becomes large and the offset fluctuates.
Such problems of dispersion of the parasitic capacitances in the interconnection parts of the fixed electrodes caused by process dispersion of the sensor and dispersion of the parasitic capacitances between the wires are not limited to differential capacitance type sensors and arise in capacitance-detecting type semiconductor physical quantity sensors in general. That is, in detecting capacitances between moving electrodes and fixed electrodes, dispersion of the above-mentioned parasitic capacitances affects the detected capacitances and gives rise to offset.
This invention has been conceived in view of the background thus far described and its first object is to provide a semiconductor physical quantity sensor with which it is possible to obtain a stable sensor output even if the usage environment changes.
It is a second object of the invention to make it possible in a capacitance-detecting type semiconductor physical quantity sensor to minimize offset of the sensor even when parasitic capacitances of interconnection parts of fixed electrodes fixed to the edge of an opening in a supporting substrate vary due to process dispersion of the sensor.
And it is a third object of the invention to make it possible in a capacitance-detecting type semiconductor physical quantity sensor to minimize sensor offset dispersion, by reducing parasitic capacitances between a wire for moving electrodes and wires for fixed electrodes.
A first aspect of the invention provides a semiconductor physical quantity sensor for detecting a physical quantity on the basis of a displacement of a moving electrode relative to a fixed electrode caused by action of the physical quantity, made up of a supporting substrate and a semiconductor substrate disposed on the supporting substrate for sensor elements and formed into a bridge structure having a bridge-like weight part and a moving electrode provided on the weight part and a cantilever structure having a cantilevered fixed electrode disposed facing the moving electrode, wherein the width of a root portion of the cantilevered fixed electrode at the fixed end thereof is narrower than the width of the fixed electrode.
When the supporting substrate warps due to thermal stress or the like, because the width of the root portion of the fixed electrode has been made narrow, transmission of the warp of the supporting substrate to the cantilevered fixed electrode is suppressed. By this means, positional deviation of the moving electrode and the fixed electrode is prevented and fluctuation of the sensor output can be suppressed. And in this way it is possible to obtain a stable sensor output even if the usage environment changes.
According to a second aspect of the invention, a change of the relative positioning of the moving electrode and the fixed electrode is detected as a change in a capacitance between the two electrodes.
When the capacitance approach is employed, a parasitic capacitance of an interconnection of the fixed electrode is formed between the fixed electrode cantilever bridge structure and the supporting substrate; however, according to the invention it is possible to reduce the parasitic capacitance pertaining to a deviation in the relative position relationship between the fixed electrode cantilever structure and the supporting substrate. As a result, an improvement in offset can be achieved.
According to a third aspect of the invention, if the width of the root portion at the fixed end of the cantilevered fixed electrode is made not more than xc2xd of the width of the fixed electrode proper, error of the sensor output can be made small.
In a fourth aspect of the invention, first and second fixed electrode pairs are each made up of first and second facing electrodes disposed facing a moving electrode part over an opening in a supporting substrate and first and second interconnection parts fixed to the supporting substrate and supporting the first and second facing electrodes, and in each of the these fixed electrode pairs the first interconnection part and the second interconnection part are electrically independent from each other and disposed facing each other on opposite sides of the opening in the supporting substrate.
According to this aspect of the invention, in each of the first and second fixed electrode pairs, a pair of electrically independent interconnection parts are disposed facing each other across the opening in the supporting substrate. Consequently, when due to process dispersion of the sensor there is positional misalignment of the opening in one direction from its predetermined position, in each of the fixed electrode pairs, the parasitic capacitance of the interconnection parts for example increases on the first interconnection part side and decreases on the second interconnection part side.
And because in each of the fixed electrode pairs the parasitic capacitance of the interconnection parts as a whole is the sum of the parasitic capacitances of the first and second interconnection parts, the amounts of the above-mentioned increase and decrease cancel out, and compared to a case wherein the opening is not positionally misaligned, effectively, dispersion of the parasitic capacitances of the interconnection parts can be reduced. Thus, according to this aspect of the invention, because positional misalignment of the opening in one direction caused by process dispersion of the sensor can be tolerated, even if the parasitic capacitances of the individual interconnection parts of the fixed electrode pairs vary, the offset of the sensor can be minimized.
In a fifth aspect of the invention, in fixed electrodes having parts fixed to edges of the supporting substrate at the opening constituting interconnection parts for extracting signals to outside, voids where portions of the interconnection parts have been removed so that the supporting substrate is exposed are formed in parts of the interconnection parts overlapping with the supporting substrate.
As a result, because the interconnection areas of the interconnection parts themselves can be made smaller by an amount corresponding to the voids than interconnection parts of fixed electrode parts in related art, even when the positional deviation or shape deviation of the opening caused by process dispersion occurs or the dispersion of the thickness of the insulating layer of the supporting substrate occurs, changes in the parasitic capacitances at the interconnection parts can be made small. Therefore, with this aspect of the invention, even if the parasitic capacitances of the interconnection parts of the fixed electrodes vary due to process dispersion of the sensor, the offset of the sensor can be minimized.
In a sixth aspect of the invention, in a capacitance detecting type semiconductor physical quantity sensor, a moving electrode pad to which is connected a wire for electrically connecting the moving electrode part to an external part is formed on the supporting substrate on a first side of the opening and fixed electrode pads to which are connected wires for electrically connecting the fixed electrode parts to the external part are formed on the supporting substrate on a second side of the opening, facing the first side.
As mentioned above, generally a capacitance C is given by C=xcex5xc2x7S/d. In this aspect of the invention, because the moving electrode pad and the fixed electrode pads are disposed facing each other on opposite sides of the opening in the supporting substrate, compared to a case wherein as in the related art both the moving electrode pad and the fixed electrode pads are disposed on the same side of the opening, the distances between the wires connected to the moving electrode pad and the fixed electrode pads can be greatly increased. Consequently, the parasitic capacitances between the wire for the moving electrode part and the wires for the fixed electrode parts can be reduced, and offset dispersion of the sensor can be minimized.
When the present inventors carried out an investigation into the relationship between sensor offset and the distances between the wire for the moving electrode part and the wires for the fixed electrode parts, the results shown in FIG. 34B were obtained. A seventh aspect of the invention is based upon these results, and provides a capacitance-detecting semiconductor physical quantity sensor having a moving electrode wire serving a moving electrode part and fixed electrode wires serving fixed electrode parts, wherein the moving electrode wire is separated from the fixed electrode parts by a distance of at least 80xcexcm.
According to studies carried out by the present inventors, as the sensitivity of the sensor increases, it is desirable that the offset (output error) be not greater than 10%. Here, if the distance between the moving electrode wire and the fixed electrode wires is made at least 80xcexcm and preferably 100xcexcm or more, even if there is dispersion or fluctuation in the distance between the wires, offset can be kept to within 10%.